This invention relates to a cascade-type scrubber, with which the wet scrubbing of the exhaust gas and removal of droplets from the gas can be performed in one and the same scrubber, producing a pure, dropless gas. It is thus essential that the cascade tubing, typical of the cascade scrubber and the equipment for droplet removal are combined into a single compact entity, which is formed of several nested chambers.
Cleaning of gases can in principle be divided into two stages, the actual scrubbing and the subsequent droplet separation. One scrubber type used extremely often is the venturi scrubber and another is the cascade scrubber.
In a cascade scrubber (e.g. Bauart BAHCO), the gas to be scrubbed is sucked in reduced pressure into a series of cascade tubes. The gas is injected via the central cascade tube into the scrubbing liquid and sucked from there upward through an annular tube around the central tube. The gas, which has taken the liquid with it, is allowed to collide with a plate surrounding the central tube, which directs the gas downward again. Thus the xe2x80x9cself-droplet formationxe2x80x9d of the liquid is achieved, and as a result the final scrubbing of the gas occurs. Droplet separation takes place in a separate droplet separator.
Different types of droplet separators are described in the literature e.g. H. Brauer, Y. B. G. Varma: xe2x80x9cAir Pollution Control Equipmentxe2x80x9d, Springer-Verlag 1981. pp. 219-239. The main types of droplet separators are the zigzag channel, centrifugal or classic filter principles.
According to this invention a compact cascade scrubber is formed essentially of a cylindrical apparatus, where the gas scrubbing equipment is located in the outer section and the droplet separation equipment in the inner section. The gas scrubbing equipment is composed of several annular chambers as is the droplet separation equipment. The gas to be scrubbed comes into the outer circle of the upper section of the scrubber from one channel and the pure, dry gas is vented through one channel from the central section of the scrubber. The chamber located in the lower section of the outer circle of the scrubber contains several scrubbing units working on the cascade principle and around it and inside it are more annular chambers belonging to the scrubber. The essential features of the invention will become apparent in the attached claims.
The advantage of having several cascade scrubbing units is that changes in the volume of gas can be reacted to swiftly. In this case, when gas is distributed from a integrated gas space to several cascade tubes, it is extremely easy to shut off some of the tubes in order to optimize conditions in the cascade tubes when the volume of gas decreases. The corresponding capacity adjustment in the droplet separator can be made in the incoming tangential channels either by adjusting the surface area of the channels or by shutting off some of the channels completely. It is well known that separating capacity (centrifugal force) only improves when tangential intake velocity is increased.
In the embodiment of the invention all the gas is conducted first to a common chamber, from where it is divided into several scrubbing units. Naturally successful distribution depends on the construction of the distribution chamber. One distribution method, well known from air-conditioning technology, is changing the cross-section of the flow in the flow direction. A typical method is to use a sloping bottom in the annular distribution chamber. It is also advantageous for the droplet separator if the scrubber has only one discharge channel, in other words, the sub-flows of gas from the cascade tubes are combined after scrubbing at least twice. The first time is immediately after the scrubbing in the cascade tubes in the annular chamber situated below the gas distribution chamber. After combining the gas flows the gas is separated in the next annular chamber into sub-flows, which are then recombined to flow into the droplet separator.
Symmetry and the use of essentially cylindrical walls are typical of a scrubber according to this invention. This is significant especially for reasons of strength, because a reduced pressure is used in most scrubbers, and the above-mentioned properties prevent the collapse of the scrubber when using reduced pressure. The actual scrubbing occurs best in a reduced pressurexe2x80x94i.e. the gas can reach a suitable velocity with a blower located after the scrubber. It is clear that scrubbing also functions under pressure, but it is not beneficial for the blower to feed hot and dirty gas through it
In the apparatus according to the present invention, the cascade scrubbing units are similar and placed symmetrically, thus aiming at optimal gas distribution. If some of the scrubbing units are shut, it is also beneficial to do this symmetrically. The scrubbed gas in the cascade tubes is conducted tangentially along several channels to the following inner, annular space, where the sub-flow of gas are combined.